For steel parts used in motor vehicles, various industrial machines, etc., a case hardening heat treatment such as carburizing-quenching, induction hardening, nitriding, or nitrocarburizing is applied to improve their mechanical properties such as fatigue strength, wear resistance, and seizure resistance. The nitriding process and the nitrocarburizing process both use a heat treatment in the ferrite region at a heating temperature not more than the A1 temperature without utilizing phase transformation. As a result, heat treatment-induced distortion can be reduced. For this reason, the nitriding process or the nitrocarburizing process is frequently used for parts having high dimensional accuracy and large parts, examples of which include gears used in automotive transmission parts and crankshafts used in engines. In particular, the nitriding process requires fewer types of gas for the process than the nitrocarburizing process, so that atmosphere control therefor is easier.
Examples of nitriding processes include the gas nitriding process, the salt bath nitriding process, and the plasma nitriding process. For automotive parts or the like, the gas nitriding process, which has high productivity, is widely employed. The gas nitriding process can result in formation of a compound layer having a thickness of 10 μm or more on the surface of the steel material. The compound layer contains nitrides such as Fe2-3N and Fe4N, and the hardness of the compound layer is much higher than that of the base metal of the steel part. Thus, the compound layer enhances the wear resistance and surface fatigue strength of the steel part at an early stage of use.
However, the compound layer has low toughness and low deformability and therefore is more likely to experience delamination or cracking during use. For this reason, nitrided parts processed by gas nitriding are not suitable for use as parts that can be subjected to impact stresses or high bending stresses. Furthermore, in the gas nitriding process, although heat treatment-induced distortion is reduced, straightening is sometimes necessary for long parts such as shafts and crankshafts. In such an instance, depending on the thickness of the compound layer, cracking may occur during straightening and this can decrease the fatigue strength of the part.
Accordingly, there is a need for a gas nitriding process that can provide a thinner compound layer or even eliminate the compound layer. By the way, it is known that the thickness of the compound layer can be controlled by the process temperature of the nitriding process and the nitriding potential KN determined by the following formula using the NH3 partial pressure and H2 partial pressure.KN=(NH3 partial pressure)/[(H2 partial pressure)3/2]
By lowering the nitriding potential KN, it is possible to provide a thinner compound layer or even to eliminate a compound layer. However, when the nitriding potential KN is low, ease of nitrogen penetration into the steel is reduced. In such an instance, the hardened case referred to as a nitrogen diffusion layer will have reduced hardness and reduced depth. As a result, the nitrided parts will have reduced fatigue strength, wear resistance, and seizure resistance. Another technique to eliminate the compound layer is, for example, machine grinding or shot blasting of the nitrided parts after the gas nitriding process. However, this technique results in higher production cost.
To respond to these problems, one proposed technique is to control the atmosphere for the gas nitriding process using a nitriding parameter, KN′=(NH3 partial pressure)/[(H2 partial pressure)1/2], which is different from the above-mentioned nitriding potential, and to thereby form a hardened case having a uniform depth (e.g., Patent Literature 1). Another proposed technique is to use, in the nitrogen penetration process, a jig having a surface made of a non-nitridable material for placement of a workpiece to be nitrided in the treatment furnace (e.g., Patent Literature 2).
By using the nitriding parameter proposed by Patent Literature 1, it is possible to inhibit the formation of the compound layer on the outermost surface in a short time. However, sometimes, sufficient hardened case depth cannot be obtained for certain characteristics required. Further, when a non-nitridable jig is prepared to perform a fluorination process as proposed in Patent Literature 2, there are additional problems such as selection of a jig and increased man hours.